Automobiles and trucks receive exterior finishes for several well known reasons. First, such finishes provide barrier protection against corrosion. Second, consumers prefer an exterior finish having an attractive aesthetic finish, including high gloss and excellent DOI (distinctness of image).
A typical automobile steel panel or substrate has several layers of finishes or coatings. The substrate is typically first coated with an inorganic rust-proofing zinc or iron phosphate layer over which is provided a primer which can be an electrocoated primer or a repair primer. Optionally, a primer surfacer can be applied to provide for better appearance and/or improved adhesion. A pigmented basecoat or colorcoat is next applied over the primer. A typical basecoat or colorcoat comprises a pigment, which may include metallic flakes in the case of a metallic finish. In order to protect and preserve the aesthetic qualities of the finish on the vehicle, it is well known to provide a clear (unpigmented) topcoat over the colored (pigmented) basecoat, so that the basecoat remains unaffected even on prolonged exposure to the environment or weathering.
Coating compositions comprise one or more film-forming polymers. Most commonly, linear polymers are employed that cure, upon application, by reaction with crosslinking agents. However, the use of non-linear graft copolymers has been disclosed. For example, U.S. Pat. No. 4,801,653 to Das et al. describes the use of hydroxy functional graft copolymers. Das et al. disclose grafting by a condensation reaction between epoxy groups of a glycidyl ester, contained in an acrylic polymer, and carboxy groups on at least a portion of vinyl monomers which are polymerized in the presence of the acrylic polymer. Such graft copolymers were found to exhibit excellent sag resistance without the necessity for externally added rheology control.
In preparing graft polymers in general, various living polymerization methods have been disclosed for obtaining functional ended polymers by selective termination of living ends. Such functional ended polymers may subsequently be attached to another polymer, that is, as so-called macromonomer "arms" on a polymeric backbone to form a comb graft copolymer. Webster, in "Living Polymerization Methods," 251 SCIENCE 887 (22 Feb. 1991) generally discloses living polymerization methods for preparing architectural forms of polymers, including graft and comb copolymers.
U.S. Pat. No. 4,680,352 to Janowicz et al. and U.S. Pat. No. 4,722,984 to Janowicz disclose the use of cobalt (Co) chelates as chain transfer agents in free radical polymerization. The latter patent discloses that macromonomers prepared by cobalt chain transfer can be polymerized to produce graft copolymers which are useful in coating and molding resins, including high solid finishes and aqueous or solvent based finishes, although the use of such polymers have so far found only limited use in the finishes area. The use of chain transfer agents to prepare dispersed polymers used in automotive coatings is disclosed in U.S. Pat. No. 5,010,140. Such dispersions are disclosed to be useful for modifying the rheology and/or the physical properties of coatings.
The present invention relates to coating compositions involving isocyanate chemistry. The evolution towards low volatile organic content (VOC) coatings has led to the need to develop new products with desirable properties. For example, with regard to isocyanate systems, it is now even more desirable to solve the problem of poor pot life and initial hardness and to achieve an optimum balance of these properties.